We’ve known that the universe is expanding ever since the work of Edwin Hubble in the early 20th century, but it was only in 1998 that we realized that the expansion of the universe is accelerating. By comparing the true luminosity to the observed brightness of distant supernovae, astronomers discovered the accelerated expansion, known as dark energy.

We’re still not exactly sure what dark energy is, but it can be understood as a property of the vacuum of space-time itself. Its density remains constant; the more empty space you have, the more dark energy you have. This unique property can accelerate expansion on cosmic scales.

However, this wasn’t always the case. Today dark energy accounts for about 70 percent of the total amount of mass and energy in the universe, with matter (especially dark matter) making up the remainder. But billions of years ago, the universe was much smaller. With a smaller volume, there was less total dark energy, and all the matter was squeezed into a tighter volume, increasing its density.

Until about 5 billion years ago, the expansion of the universe was actually decelerating as it was dominated by the gravitational attraction of matter. But in the background, dark energy remained constant; once the matter diluted itself over too much volume, the era of accelerated expansion began.

This acceleration persists to today and — unless some new understanding radically changes our picture of dark energy — will persist perpetually into the future.